1. Field of the Invention
The present invention relates to an image heating apparatus that heats an image on a sheet. The image heating apparatus is used in image forming apparatuses such as a copying machine, a printer, a facsimile, and a multifunction apparatus having some of functions of these machines.
2. Description of the Related Art
In an image forming apparatus, a toner image (developing-agent image) is formed on a sheet and is fixed on the sheet by being heated and pressed with a fixing device serving as an image heating apparatus. From the recent viewpoint of energy saving, Japanese Patent Laid-Open No. 2009-92785 proposes a fixing device using a heating belt having a heating resistor layer as a fixing device that provides high heat transfer efficiency and a quick temperature rise. Since the heating belt itself generates heat by energization of the heating resistor layer in this method, heat of the heating belt can be efficiently transferred to the sheet.
In the heating belt adopted in the fixing device described in Japanese Patent Laid-Open No. 2009-92785, the heating resistor layer is stacked on a cylindrical insulating base material and electrode layers are stacked at both widthwise ends of the heating resistor layer. Power feed members formed by carbon chips or the like are pressed against the electrode layers of the heating belt. The power feed members feed power to the electrode layers of the rotating heating belt while sliding on the electrode layers. When current thus flows to the heating resistor layer of the heating belt, the heating resistor layer generates heat, and the heating belt is entirely heated.
Since the power feed members and the electrode layers slide on each other in this fixing device, the electrode layers wear down along with the use of the fixing device. The progress of wear of the electrode layers shortens the life of the heating belt.
To overcome this problem, in a fixing device disclosed in Japanese Patent Laid-Open No. 2014-232302, electrode layers are reinforced by attaching ring-shaped members to end portions of a heating belt. Specifically, in this fixing device, ring-shaped members disposed along an outer periphery of the heating belt and ring-shaped members disposed along an inner periphery of the heating belt clamp the heating belt therebetween to fix the ring-shaped members to the heating belt. At this time, the outer ring-shaped members are electrically connected to the electrode layers. This allows power feed members to feed power to the electrode layers via the ring-shaped members.
However, when the ring-shaped members are attached to the heating belt, the heating belt sometimes wears down according to the structure of the ring-shaped members. The cause of occurrence of wear of the belt will be described with reference to FIGS. 11A and 11B. FIG. 11A schematically illustrates the structure of a fixing device in which ring-shaped members are attached to one longitudinal end portion of the belt. FIG. 11B shows cross-sectional views taken along A-A and B-B of FIG. 11A.
As illustrated in FIG. 11A, in the fixing device, a nip portion N is formed by abutment of the belt and a pressing roller. For this reason, as illustrated in the B-B cross section of FIG. 11B, the cross section of a longitudinal center portion of the belt is locally flat at the nip portion N. On the other hand, in the fixing device, as illustrated in FIG. 11A, front and back sides of the longitudinal end portion of the belt are clamped between an inner ring-shaped member and an outer ring-shaped member. For this reason, the cross section of the longitudinal end portion of the belt is kept circular, as illustrated in the A-A cross section of FIG. 11B. At this time, the height of the belt on the side of the nip portion N is different by a height H between the longitudinal center portion and the longitudinal end portion of the belt.
As illustrated in FIG. 11A, in a section D of the belt in the longitudinal direction, the belt is deformed to connect the A-A cross section and the B-B cross section. Since an inner surface of the belt strongly abuts on a corner portion of the inner ring-shaped member in such a state, local stress concentration occurs. In a portion of the belt where stress concentrates, wear is promoted and the strength decreases, which shortens the life of the belt. Therefore, in the fixing device in which the belt for generating heat by energization is reinforced by the ring-shaped members, the life of the belt is to be suppressed from being shortened by the abutment between the corner portion of the inner ring-shaped member and the inner surface of the belt.